Plant treatment compounds, including foliar-applied herbicides, have often been formulated as concentrates with a surfactant. When water is added to such a concentrate, the resulting sprayable composition is, by virtue of the surfactant provided therein, more easily and effectively retained on foliage (ie., leaves and other photosynthesizing organs) of plants. Surfactants can also provide other benefits, including improved contact of spray droplets with a waxy leaf surface and, in some cases, improved penetration of the accompanying plant treatment compound into the interior of leaves. Through these and perhaps other effects, particular surfactants have long been known to increase the biological effectiveness of herbicide compositions, or other compositions of plant treatment compounds, when present in such compositions.
Thus, for example, the herbicide glyphosate (N-phosphonomethylglycine), usually in the form of a water-soluble salt thereof, has been formulated with surfactants such as those having polyoxyalkylene (polyoxyethylene and/or polyoxypropylene) moieties, including, among other surfactants, polyoxyalkylene alkylamines.
The term "alkyl" as an element in the description of a surfactant herein is used in the sense in which it is conventionally used in surfactant-related art to embrace unsaturated as well as saturated hydrocarbyl chains, and includes linear and branched chains. In general, alkyl groups useful as hydrophobic moieties in surfactants contain about 8 to about 22, most commonly about 12 to about 18, carbon atoms.
Surfactants have been combined with glyphosate or other plant treatment compounds in (a) liquid or solid concentrate compositions provided by the supplier and diluted, dissolved or dispersed in water by the user before application, (b) ready-to-use dilute aqueous compositions provided by the supplier and applied without further dilution by the user, and (c) user-prepared dilute aqueous compositions made by adding separate compositions of plant treatment compound (e.g., glyphosate) and surfactant to water prior to application. Such user-prepared dilute aqueous compositions are known as "tank-mix" compositions.
Some surfactants, although chemically stable, are physically incompatible with certain plant treatment compounds, particularly in aqueous liquid concentrate compositions. For example, an aqueous micellar solution of most classes of nonionic surfactant, including polyoxyethylene alkylether surfactants, does not tolerate the presence of high concentrations of salts imparting high ionic strength to the solution. This is true, for example, of a polyoxyethylene alkylether surfactant in a concentrated aqueous solution of a salt of glyphosate. Such physical incompatibility can be manifested immediately on preparation of the composition, or over time or when exposed to certain temperature conditions or regimes, leading to inadequate shelf-life of the composition. A common effect of physical incompatibility is separation of the composition into distinct phases. Other problems that can arise from such incompatibility include formation of aggregates large enough to interfere with commercial handling and application, for example by blocking spray nozzles.
Many plant treatment compounds are commercially packaged as a liquid concentrate formulation that, while being a concentrate, nevertheless contains a significant amount of water. The packaged concentrate, containing a plant treatment compound as active ingredient, is shipped to distributors or retailers. Ultimately, the packaged concentrate is purchased by an end user, who dilutes the concentrate by adding water in accordance with label instructions on the package. The fully diluted material is then sprayed on plant foliage.
A significant portion of the cost of such a packaged concentrate is the cost of transporting the concentrate from the manufacturing site to the location where the end user purchases it. Any liquid concentrate formulation that contains relatively less water and thus more active ingredient would reduce the cost per unit amount of active ingredient. However, one important limit on the ability of the manufacturer to increase concentration (ie., "loading") of the active ingredient in a concentrate formulation is the physical stability of that formulation. With some combinations of formulation ingredients, an upper limit to loading of the active ingredient is reached beyond which any further reduction of water content in the concentrate causes physical instability (e.g., separation into discrete layers), which generally makes the concentrate commercially unacceptable.
Until now, the problems presented by such physical instability have been avoided by such expedients as (a) formulating the concentrate in solid rather than liquid form, (b) restricting surfactant choice to the relatively narrow range of surfactants showing good physical compatibility with high ionic strength solutions, or (c) limiting the concentration of a surfactant such as a polyoxyethylene alkylether to a low level, often in combination with a second surfactant or other coformulant that acts as a compatibility agent. Illustrations of such expedients are provided by literature on glyphosate formulations.
It is known to include a relatively high concentration of a polyoxyethylene alkylether surfactant together with a glyphosate salt in a solid formulation. For example, U.S. Pat. No. 4,931,080 discloses a solid powder composition containing the polyoxyethylene alkylether surfactant Plurafac.TM. A-39 of BASF and glyphosate trimethylsulfonium salt, in a weight ratio of 1:1.64. The weight ratio of surfactant to glyphosate acid equivalent (a.e.) in this composition can be calculated to be about 1:1.13. Two other compositions are disclosed having the same ingredients in an even higher weight ratio of surfactant to glyphosate a.e. (1:0.86 and 1:0.69).
Solid formulations such as those disclosed in above-cited U.S. Pat. No. 4,931,080 have numerous benefits but most end users prefer the convenience of liquid formulations. Further, the compositions of U.S. Pat. No. 4,931,080 containing Plurafac.TM. A-39 are disclosed therein to be less herbicidally effective than a comparative liquid formulation having a surfactant identified as "Ethoquad 12" (sic; possibly Ethoquad.TM. 18/12 of Akzo) at a weight ratio of surfactant to glyphosate a.e. of 1:1.71. Thus polyoxyethylene alkylether surfactants such as Plurafac.TM. A-39 are shown in the art to be relatively weak enhancers of glyphosate herbicidal effectiveness when used at high surfactant to glyphosate a.e. ratios (1:1.13 and higher).
In the art of making aqueous liquid concentrate formulations of salts of glyphosate such as the isopropylammonium and trimethylsulfonium salts, where the formulations contain a substantial quantity of surfactant, cationic surfactants such as polyoxyethylene alkylamines and quaternary ammonium salts, amphoteric surfactants such as polyoxyethylene alkylamine oxides, and a very limited range of nonionic surfactants such as alkyl polyglucosides that exhibit good physical compatibility with high ionic strength solutions, have hitherto been preferred. Where polyoxyethylene alkylether surfactants have been included in an aqueous concentrate formulation of a glyphosate salt, they have generally been present in relatively low concentration and/or as minor components of a blend of surfactants in which more compatible types such as quaternary ammonium salt or alkyl polyglucoside surfactants predominate. Further, the polyoxyethylene alkylethers used in this way have in general been of relatively low molecular weight, having a hydrophobic moiety consisting of a relatively short (e.g., C.sub.9-15) hydrocarbon chain and/or having a hydrophilic moiety consisting of a relatively short polyoxyethylene chain (e.g., 3-10 oxyethylene units).
Illustrative examples of such formulations can be found in International Patent Publication No. WO 95/16351, wherein the polyoxyethylene alkylether is an ethoxylated secondary alcohol such as Tergitol.TM. 15-S-9 of Union Carbide, in which the hydrophobic moiety is a C.sub.11-15 hydrocarbon chain and the hydrophilic moiety is a polyoxyethylene chain having on average about 9 oxyethylene units. In order to provide a stable aqueous concentrate, the polyoxyethylene alkylether surfactant is accompanied by, for example, a cationic surfactant such as polyoxyethylene (2) N-methyl cocoammonium chloride.
Additional illustrative examples can be found in U.S. Pat. No. 5,464,806, wherein the polyoxyethylene alkylether is an ethoxylated acetylenic diol such as Surfynol.TM. 465 of Air Products, in which the hydrophobic moiety is a branched C.sub.14 hydrocarbon group that is symmetrical about an acetylenic triple bond and the hydrophilic moiety comprises a pair of polyoxyethylene chains. Various cosurfactants for the ethoxylated acetylenic diol are disclosed that can help provide a stable aqueous concentrate formulation. These include cationic surfactants such as Ethoquad.TM. C/12W, Ethoquad.TM. 18125 and Ethomeen.TM. T/25 of Akzo and Emcol.TM. CC-9 of Witco, and a nonionic surfactant "APG 325" (Agrimul.TM. PG 2069 of Henkel).
As indicated above, the prime motivation for including a surfactant in a formulation of a foliar-applied plant treatment compound such as glyphosate is to enhance biological effectiveness of the plant treatment compound. This motivation has often led to the use of relatively large amounts of surfactant in relation to the amount of active ingredient, for example a weight ratio of surfactant to active ingredient of at least about 1:2, in many cases about 1:1 or even higher. An enduring problem in the art is that the more surfactant that has to be included in an aqueous concentrate formulation, the lower is the maximum active ingredient loading that can be achieved.
Co-assigned U.S. patent application Ser. No. 08/957750, the pertinent disclosure of which is incorporated herein by reference, teaches that biological effectiveness of foliar-applied plant treatment compounds or exogenous chemicals, including glyphosate, is enhanced to a surprising degree by inclusion in a composition thereof, at a lower weight ratio of surfactant to glyphosate a.e. than previously disclosed, an alkylether or alkenylether surfactant having formula I: EQU R.sup.12 --O--(CH.sub.2 CH.sub.2 O).sub.n (CH(CH.sub.3)CH.sub.2 O).sub.m --R.sup.13 I
wherein R.sup.12 is an alkyl or alkenyl group having about 16 to about 22 carbon atoms, n is a number whose average is about 10 to about 100, m is a number of 0 to an average of about 5, and R.sup.13 is hydrogen or a C.sub.1-4 alkyl group. The weight ratio of such polyoxyethylene alkylether or alkenylether surfactant to plant treatment compound is about 1:3 to about 1:100. The high degree of herbicidal effectiveness of compositions of U.S. patent application Ser. No. 08/957750 is especially surprising in view of the relatively poor results previously reported for higher weight ratios of surfactant to glyphosate a.e. in above-cited U.S. Pat. No 4,931,080.
Preferably, in a surfactant of formula I, R.sup.13 is hydrogen and m is 0, thus a preferred class of alkylether or alkenylether surfactants of U.S. patent application Ser. No. 08/957750 has formula II: EQU R.sup.12 --O--(CH.sub.2 CH.sub.2 O).sub.n H II
wherein R.sup.12 is an alkyl or alkenyl group having about 16 to about 22 carbon atoms and n is a number whose average is about 10 to about 100.
Clearly, from this teaching, it would be desirable to make a highly loaded concentrate formulation of a plant treatment compound and a polyoxyethylene alkylether surfactant wherein the weight ratio of such surfactant to the plant treatment compound is relatively low, for example no higher than about 1:2, or no higher than about 1:3. Such formulations are indeed disclosed in above-cited U.S. patent application Ser. No. 08/957750, but further investigation has shown these formulations to lack the degree of shelf-stability required for commercial acceptability. In particular, they exhibit phase separation when stored for periods of a month or more, in some cases for periods as short as 24 hours, especially under temperature conditions that vary in the range from about -10.degree. C. to about 50.degree. C., as are experienced by agricultural chemical formulations in normal commercial use.
Among the plant treatment compounds whose biological effectiveness is taught by U.S. application Ser. No. 08/957750 to be enhanced are compounds that are acids of relatively low solubility in water, for example below about 50 grams per liter (g/l). Such compounds are almost universally formulated as water-soluble salts. In the case of glyphosate, for example, aqueous compositions are typically prepared using an alkali metal (e.g., sodium or potassium) salt, an ammonium salt, an alkylammonium (e.g., dimethylammonium or isopropylammonium) salt, an alkanolammonium (e.g., monoethanolamine) salt or an alkylsulfonium (e.g., trimethylsulfonium) salt.
The particular instability problem referred to above relates to the fact that surfactants of formula II, when placed in an aqueous solution of high ionic strength, such as a concentrated solution of a plant treatment compound in salt form, tend to separate from the solution so that two discrete phases are formed, one of which consists primarily of the surfactant and one that is primarily aqueous and typically contains most of the dissolved plant treatment compound. In some cases it is possible to disperse such a surfactant, in the presence or absence of an oil such as butyl stearate, in the solution, so that the surfactant remains dispersed for at least a few hours or days at ambient temperature. However, such dispersions are rarely physically stable for prolonged periods, especially over a range of storage temperatures as are typically experienced by commercial agricultural chemical formulations in normal use.
U.S. patent application Ser. No. 08/957750 teaches that greater stability can be imparted to an aqueous concentrate composition containing a dispersed surfactant of formula II and a plant treatment compound in salt form by adding other ingredients, for example colloidal particulate silica or aluminum oxide at 0.5% to 2.5% by weight. Even with such addition, however, experience has shown that physical stability of the aqueous concentrate composition is generally insufficient to provide a commercially acceptable shelf-life.
Thus, a particular problem addressed by the present invention is to prepare a stable, highly loaded aqueous concentrate composition containing a plant treatment compound such as glyphosate and a polyoxyethylene alkylether surfactant, wherein the surfactant is present at a weight ratio to the plant treatment compound of about 1:2 or less, but in an amount sufficient, upon dilution of the composition in water and application of the diluted composition to foliage of a plant, to provide the desired enhancement of biological effectiveness of the plant treatment compound. By "highly loaded" in this context is meant a concentration of active ingredient, expressed as acid equivalent (a.e.), not less than about 240 grams per liter (g a.e./l). More desirably, the concentration of active ingredient is about 300 g a.e./l or higher, ideally as high as, say, 360-480 g a.e./l.
As aqueous concentrate compositions containing a plant treatment compound such as glyphosate and a surfactant typically have densities of about 1.1 to about 1.2 grams per milliliter (g/ml), a "highly loaded" composition as defined above can be calculated to have a concentration of active ingredient not less than about 20% by weight, more desirably about 25% by weight or higher, ideally as high as, say, 30-44% by weight.
International Patent Publication No. WO 94/19941 discloses one approach to solving this problem by which it is implied that a concentration of 5% to 58% by weight of a plant treatment compound (for example glyphosate or glufosinate) can be achieved in an aqueous composition in the presence of any of a variety of surfactants. This approach involves preparing an aqueous suspension of glyphosate or glufosinate herbicide in the presence of a concentrated solution of an electrolyte. The herbicide can be in its acid form or in the form of a salt. Alternatively, both acid and salt forms can be present. The presence of the electrolyte is stated to lower solubility of the suspended herbicide, hence reduce risk of crystal growth and thereby improve long-term stability of the suspension. Experiment no. 26 of WO 94/19941 teaches a suspension concentrate containing 20.4% by weight of glyphosate acid, 10.2% by weight of polyoxyethylene (25) stearylether (a surfactant of formula II above), 27.2% by weight of ammonium sulfate as electrolyte, 1.4% by weight of colloidal attapulgite clay and 40.8% by weight of water. No higher loading than 20.4% by weight is specifically exemplified for glyphosate acid together with a surfactant of formula II. A density of 1.27 g/ml is reported for the composition of Experiment no. 26, from which it can be calculated that the glyphosate loading was about 260 g a.e./l. It is disclosed that following storage at various temperatures, this composition exhibited an "upper clear phase" amounting to 10% of the volume of the composition.
The disclosure that follows provides a new solution for the storage stability problem of a highly loaded aqueous concentrate composition of a plant treatment compound that also contains a polyoxyethylene alkylether or alkenylether surfactant in a desirable weight ratio to the plant treatment compound.